Type A RCD for an old consumer unit.

Hasn't given me a quote yet - I'll let you know. Seems to be a reasonable person.

If you have a standard consumer unit with single pole MCBs switching the affected circuit on and off has no effect. The neutral and earth are always connected.

Dave

I agree, it doesn't explain my problem.

That is how neutral earth shorts work.

In general the nearer you are in cable-meters to the short the more likely a load is to trip it, as the neutral current has less resistance to eaeth the closer you get to the short

The more current you draw and the nearer to the short you are the more neutral current will bypass the RCD...

I trust my electrician will check all the circuits before he reconnects them to the new CU. Or at least he should!

If that is happening then that IS the best time to do it.

Also run a megger over the wiring...

I suppose I should do that now, but it's such a rat's nest that I'd probably have to turn all the power off before disentangling them - I doubt they've been done in the right order - I know for a fact some of them haven't! I'd also have to get a proper 500v tester and presumably disconnect everything from the sockets that might be damaged?

I'm with NT on this. At some part of the circuit with the sockets you switch off there is a likely earth neutral short.

The only way you're going to confirm is to disconnect either the neutral or earth circuit for these sockets at the consumer unit and check isolation between earth and neutral. A standard meter resistance measurement would be a good start.